Post
by The Scum » Sun Feb 15, 2015 12:35 am
So I don't mean to be teeriably alarmist about the high voltage there.
But I do mean that I want you to understand that it can be dangerous, and help you understand how to deal with it. But many internet posts tell you to "be safe", but fall short when it comes to telling you what actually goes into being safe.
So here's my attempt (EMRR, AshcatIT, the esteemed Mr Williams and others, please chime in and correct me if I'm giving bad advice! Seriously, in my day gig, I've mostly escaped with 5V world for the 3.3V one, and see the 1.8 V world on the near horizon.). This is not meant to be an exhaustive exploration of the subject, but a handful of things that have served me well. If this all reads as gibberish,take the device to a real repairman. I'm assuming that since you're posting here, you're aware that there's some danger, and that you're responsible for your own actions.
There's nothing like having the cold sweats while working on a high voltage circuit to make things even more complicated.
There are two situations to be aware of.
First, when the circuit is on, there's high voltage in there, and it can refill itself indefinitely. If you're poking around while it's live, the common wisdom os to use only one hand ("keep one hand in your back pocket" as the greybeards say). That way, even if you touch the live circuit, it won't ever go from your left hand to your right hand, via your heart.
I've got a set of extra long (maybe 6") banana-grabber probes that help with that situation - I can position them carefully, one at a time, with one hand, to take readings with a multimeter or scope. Similarly, grabber hooks and probes mean you can latch onto something, then adjust other things, keeping with the "one hand" principle.
Second, after it's off, those big caps might still hold some charge...and they can magically refill themselves (google for "dielectric absorption" if you don't take my word for it). So the common practice is to put a high value resistor (100K is reasonable, 1M will take longer to discharge) from the B+ line to ground (effectively across the leads of the big cap in this case).
It's a "measure twice, cut once" situation - don't guess whether the circuit is discharged!
Since you've already been poking around in there with it off, and haven't reported sparks/smoke/ozone smell, it might already have that resistor permanently installed. The way to check is to put a multimeter between B+ and ground. When you turn the circuit off, if there's a bleeder resistor, B+ should creep towards ground...a minute or so after switching off, it should read less than 1 Volt.
My opinion is that the old practice of discharging tube circuits by corwbarring B+ to ground with a screwdriver is dangerous...other folks may want to argue the other side...I much prefer having a highish resistor permanently installed. It only costs a tiny bit of current while the circuit is on, and helps keep it safe while it's off.
"What fer?"
"Cat fur, to make kitten britches."
